The present embodiments relate to phase unwrapping. In particular, aliasing of velocity data in phase contrast magnetic resonance (MR) imaging is reduced or removed.
Phase contrast MRI is often used to visualize blood moving through blood vessels. The acquisition may include at least two image sequences. Magnitude images show the anatomy of the vessels, and the phase contrast images show the velocity of the blood through those vessels. FIG. 1 shows a magnitude or intensity MR image of anatomy, above the heart in this example. FIG. 2 shows the phase contrast MR image of the same patient region. In FIG. 2, the outer grey ring 30 is tissue, the middle ring black and white region 32 is the lungs, the dark region 34 is negative velocities, and the light region 36 is positive velocities.
For phase contrast MRI, phase images with and without velocity encoding gradient are subtracted to measure a signal proportional to physical velocity. The phase difference, in one example, is represented using 12 bits in unsigned short format. The phase difference is assigned a number between 0 and 4095. This signal is then converted to physical velocity by a velocity encoding parameter (VENC). The VENC is related to the magnitude of the velocity encoding gradient. In the 12 bit phase example, the VENC is used to calculate the physical velocity so that 0 represents a velocity of −VENC, 2048 represents a zero velocity, and 4095 represents a velocity of +VENC.
When the actual velocity is higher than the VENC, the resulting estimated velocity is wrapped or aliased. For example, the VENC is chosen at 100 m/s. The maximum actual velocity (one of the actual velocities) in the aorta being imaged is 150 m/s. In this case, the gray value for the velocity should be 5119, but that is larger than 4095. Instead, the value is wrapped around and becomes 1024, which appears to be a velocity of −50 m/s after applying the VENC.
FIG. 3 shows a phase contrast MR image with this VENC aliasing. As compared to FIG. 2, the aorta region that should be light or white in the center is aliased as black. FIGS. 1-3 are shown in grayscale, but may be colorized. For example, flow in one direction is mapped to red shades and flow in another direction is mapped to blue shades. In grayscale, black or dark is mapped to negative velocities and white or light is mapped to positive velocities. In FIG. 3, the dark interior of the generally light region is due to aliasing.
This aliasing may be confusing. Measurements made based on velocity may be distorted. For example, a calculation of an average velocity inside the aorta is tainted by the wrong or aliased velocities. The VENC may be reset to a larger value, but this may be time consuming and may result in selection of an overly large VENC just to avoid aliasing. A larger VENC than needed reduces the dynamic range of the velocity information.